Abrasive grains comprised of Al2O3 are processed industrially into abrasives in large quantities due to their great hardness, chemical inertia and high temperature resistance. In this respect, so-called sintered corundum produced via a ceramic or chemical route is utilized for certain areas of application in addition to melting corundum that can be produced relatively cost-efficiently in an electrical arc furnace and which therefore accounts for the largest share of the production of abrasive materials. The advantages of sintered corundums in terms of abrasion engineering in certain grinding operations based on their microcrystalline structure have been known for approximately 50 years.
Thus, for example, polycrystalline Al2O3 bodies are described in U.S. Pat. No. 3,909,991 whose crystallite size lies in the submicron range and whose density amounts to over 95% of theoretical density. Production occurs through hot pressing from a mixture of soot and granulated Al2O3 that, according to U.S. Pat. No. 3,079,243, is obtained by crushing cold-pressed Al2O3 shaped bodies. The process is relatively expensive and poorly suitable for large-scale industrial production.
In EP-B-0 152 768, an abrasive grain is described that is produced by way of sol-gel technology at relatively low sintering temperatures. In this process, crystallization germs are added as sintering aids that accelerate the sintering process and prevent uncontrolled crystal growth. The microcrystalline abrasive grains produced in this fashion dispose of a submicron structure and are used preferably in abrasive disks with a ceramic binding for precision grinding. Even more finely structured corundum abrasive grains with a median crystallite size of 0.2 μm are obtained via the sol-gel technology according to EP-B-0 408 771.
EP-B-0 725 045 describes a method for the production of sintered α-Al2O3 bodies according to which microcrystalline abrasive grains can be produced via a conventional ceramic technology even without the relatively expensive sol-gel process.
All methods mentioned above comprise as one process phase the crushing and treatment of the green body or sintering body for abrasive granulation which is done according to conventional methods (crushers, cylinder crushers, or similar). With this type of crushing, one does not obtain a preferable grain shape, but it will lead to a grain band with the most varying grain shapes and grain sizes, and the only option is to enrich—to a limited degree—certain grain shapes and grain sizes by selecting suitable crushing aggregates.
It must be observed, however, that not only the microstructure but also the geometric shape and the size of the abrasive grain itself plays a decisive role in the abrasion process. For example, for certain areas of application in which only minor forces act upon the abrasive grain, aggressive, sharp-edged and splinter-shaped abrasive grains are preferred while in other areas of application in which, for example, the abrasive grain is subjected to high pressures, compact, cubic grains have proved to be suitable. The grain shape itself, however, can only be influenced to a certain, limited extent through conventional treatment with jaw crushers, cylinders and mills, and the yield of grains with a specific grain shape within a defined grain size is usually relatively low in relation to the total grain amount produced. On the other hand, a precisely defined grain shape and grain size is often required for the abrasion process itself in order to assure a certain amount of abrasion and surface quality. But since the grain distribution on the grain band conforms only to a limited degree to the grain sizes and shapes required for the grinding process, costly follow-up processing or additional comminution will be necessary in order to meet the actual demand. This leads to the fact that in the treatment and screening of abrasive granulation, there will always be a relatively large portion of “waste”, i.e. abrasive grains that can no longer be used in the production of abrasives. The development works in the area of abrasive granulation of recent years therefore has aimed not only at quality improvement and increased performance but also at a defined production of abrasive granulation with a certain grain shape and grain size.
For example, U.S. Pat. No. 3,387,957 describes a microcrystalline abrasive grain based on bauxite that has the shape of a small rod and that is produced through extrusion of a mixture of finely ground bauxite; a liquid, preferably water; and a binder. This way, it was not only possible to deliberately produce a defined granulation, but it also turned out that the resulting rods, as the preferred geometric shape, showed particularly good abrasive results for certain grinding operations such as rough grinding. However, the abrasive performance may be negatively influenced by the fact that its contents of aluminum oxide amounts to only approximately 85–88 weight percent, while the other components such as Fe2O3, SiO2, CaO2 and MgO do not possess the necessary physical properties to be effective as abrasives. The contents of α-Al2O3 is all the more reduced through the fact that under the sinter conditions at hand mullite (3 Al2O3×2 SiO2) forms that is of clearly lower hardness than corundum. An abrasive grain based on corundum with such a number of occlusions is therefore of little use for the processing of materials such as hardened steel, chromium steel, cast iron or similar materials that make great demands on the performance potential of the abrasive grain.
U.S. Pat. No. 4,252,544 describes an abrasive grain that has a density of more than 3.75 g/cm3 (≧90% of the theoretical value) and a Knoop hardness of more than 1,900 kg/mm2, that consists of 98% of aluminum oxide and whose structure is made up of a mixture of coarse crystals in a range of 3–10 μm and fine crystals whose median particle size is less than 2 μm. The abrasive grain is produced by mixing preferably 20 to 50 weight percent of electro-corundum with a particle size of 3 to 10 μm and calcined Bayer alum earth (preferably 80 to 50%) whose particle size lies between 0.2 and 1 μm; by adding water and a binder while kneading; by extruding the mixture and subsequently sintering the crushed extruded material. The sintering temperatures lie at 1,550° C. to 1,650° C.
While such a material disposes of clearly more favorable abrasive properties than the previously described materials, its structure is still clearly coarser as compared with the aforementioned submicron corundums produced through sol-gel technology, and its density is considerably lower, meaning that a material according to U.S. Pat. No. 4,252,544 is less tough and strong than the materials on sol-gel basis with a submicron structure.
The toughness and strength of an abrasive grain have varying effects on the abrasion process. On the one hand, great toughness and strength are needed to prevent the abrasive grain from wearing out too quickly; on the other hand, sharp cutting edges are needed for an efficient grinding process. Sharp cutting edges can be formed, on the one hand, when the worn-out and rounded grain breaks out of its binding, bringing to bear fresh grains with sharp edges from an underlying layer, or when—in a more favorable case—smaller areas are broken off the abrasive grain itself due to the forces that affect the abrasive grain during the grinding process, thereby forming fresh cutting edges without the loss of the grain. The tougher and stronger an abrasive grain is, the larger its performance potential will be, which, however, can be utilized only if one succeeds in ensuring the self-sharpening of the abrasive grain described above. In that case, the forces affecting the grain must be great enough in order to achieve a second sharpening; while at the same time, if possible, a breaking off of the grain should be avoided. However, the stronger the forces that work or rather must work on the individual grain in order to bring about a second sharpening effect, the better and more strongly the grain must be imbedded in the abrasive material.
One use in which the abrasive grain is exposed to high pressures is, for example, the rough grinding of plate slabs and billets in foundry operations or in steel mills where sinter corunds are frequently used in so-called hot-pressed disks. Synthetic resin serves as the basis to bind the grain. The disks themselves are highly compressed and almost without any pores.
If one takes a general look at the wear and tear mechanism of a conventional, coarsely crystalline abrasive grain during the abrasion process one will realize that at first the grain wears out, becoming more and more dull, in order to finally break out completely from its binding. The corunds with a microcrystalline structure now have the advantage that, after the primary edges are worn out and rounded, smaller areas of the grain itself can break off and thereby form new cutting edges that enter into the grinding process again. This process repeats itself and the abrasive grain itself breaks off from its binding much later. In other words, the grinding disk wears out less quickly and thus provides greater abrasion performance. The finer the crystals and the more densely a sinter corundum is structured, the tougher the material will be, and the smaller the areas that will break off from the grain. That means that the self-sharpening described above can repeat itself several times depending on the grain size; the cutting edges themselves will be in use longer, and thereby, at least theoretically, a greater abrasion performance should be achieved. The basic preconditions for a meaningful functioning of an efficient second sharpening is of course that the grain is of a basic strength that prevents the grain from abrading and wearing out too quickly.
But now it happens, particularly in the case of the above-mentioned grinding with hot-pressed disks, that a reverse effect makes itself felt that affects the abrasion performance negatively. If one wishes to increase the grinding performance and the serviceable life of a disk by utilizing more and more finely structured and denser grain materials, one will realize that from a certain toughness of the material on—which in turn depends on the density and the crystal structure—the forces necessary to bring about a second sharpening exceed the binding forces, meaning that the grain, as usual, will become more dull but will no longer re-sharpen itself, instead it will suddenly break out of its binding. The abrasion performance and the serviceable life of such a disk with an alleged “better” abrasive grain thus can turn out to be much lower than that of a disk with an abrasive grain of lower density and coarser structure.
If one still wishes to exploit the higher performance potential of the abrasive grain, one customarily starts by improving the imbedding of the grain. The more resistance the binding offers to a breaking off of the abrasive grain from the imbedding, the stronger the effect of the disk. But the degree of hardness of any abrasive disk can be increased only to a certain extent, and particularly with the use of synthetic resin as a basis for the binding, the optimization potential for the binding frequently does not suffice to anchor a microcrystalline, preferably submicron, dense grain so firmly that it can fully bring to bear its performance potential that, at least theoretically, should exist due to its structure.
The problem can be solved in part by using the sinter corundum not in its pure state but as a mixture together with conventional melting corundum. This method has proved its value especially with the above-mentioned sol-gel corundums that are usually utilized as a mixture with conventional corundum types in padded abrasive materials as well as in abrasive disks. Put in simple terms, in such a combination the conventional corundum provides the necessary cutting power while the sinter corundum accounts for the serviceable life of the material and its abrasive performance.
EP-B-0 395 091 describes sintered fibrous sol-gel abrasive grains whose crystallite size preferably lies below 1 μm and that are produced by extruding a Böhmite gel with a solid contents of preferably between 45–64 weight percent. Similar to the case of the afore described rod-shaped corundums, it is also possible in the case of submicron sol-gel corundums to drastically increase the yield of merchantable graining in a methodical fashion. In addition, the material shows even better abrasion performance in certain grinding operations due to its grain shape than a comparable sol-gel corundum that was crushed through conventional methods and that does not dispose of any preferable shape.
However, if one wishes to assess the performance potential and the usefulness of a certain abrasive grain it is, in principle, unavoidable to define the area of use for the abrasive grain as well, since depending on the area of application the most varied demands are made on an abrasive grain; those demands depend, among other things, for example on what material is to be treated with what objective (surface quality, abrasion, etc.). In practice, nearly all steel types, metals, alloys, the most varied types of woods, stone, glass, synthetics, lacquers and many more are being treated with abrasives. Even though one tries to use, as universally as possible, an abrasive grain that possesses the relevant properties for the grinding process such as great hardness, toughness, chemical and thermal resistance and many others more, there occur, time and again, shifts in the assessment of the individual abrasive grain types among each other since, depending on the area of application, certain properties of a grain such as toughness, hardness, brittleness, thermal resistance and/or chemical resistance, are of greater importance. Add to this the economic aspect. For example, it does not make much sense to treat a material that is relatively easy to process with rather expensive materials with a high performance potential if their strong performance does not come to bear at all in a particular case and if a considerably less expensive grain shows a similar abrasion result.
For example, in spite of their potentially very great general performance potential, there are several reasons for not using rod-shaped microcrystalline corundums made according to EP-B-0 395 091 in hot-pressed disks. The abrasive grain described in EP-B-0 395 091 has an extraordinarily fine structure and is very dense, meaning that a re-sharpening requires extraordinarily strong forces that under certain circumstances can not be compensated through an adaptation of the imbedding; one must therefore assume a relatively high attrition of the disk due to the breaking off of grains. In addition, its very application in rough grinding where the known, relatively inexpensive sinter corundums show good abrasion performance, must be put in question for economical reasons since the production costs of an abrasive grain made according to EP-B-0 395 09 lie clearly above those of conventional sinter corundums due to the expensive raw materials and costly sol-gel process even if it may have been possible to significantly increase the yield of a particular grain by way of process management.